Etymologies

Middle English rarefien, from Old French rarefier, from Medieval Latin rārificāre, alteration of Latin rārēfacere : rārus, rare + facere, to make; see dhē- in Indo-European roots.

(American Heritage® Dictionary of the English Language, Fourth Edition)

rare +‎ -fy (Wiktionary)

Examples

I'm no stranger to the Gentri-verse and I ride my bike all the time, and I can't even relate to him--which, I guess, is why I also can't always relate to the "bike culture," since they seem so determined to rarefy the normal.

Because the elements, on the other hand, can be rarefied and condensed, they can incline the lumen in themselves away from the center of the universe, so as to rarefy it, or toward the center so as to condense it, and this accounts for their natural capacity to move up and down.

The arid plains from which the conglomerate crops out rarefy the superincumbent air-stratum to such a degree that the intensely chilled layers resting on the closely adjoining snow-peaks pour down to reëstablish equilibrium, with the wrathful force of an invisible cataract, eight, ten, even seventeen thousand feet in height.

I shall protest the roving apeman tribes, the sheep-people munching the far fields prayed on by the feudal land-baron wolves who rarefy themselves in the few skyscraper summits and horde unremembered foods.

It is certainly possible to take a hollow core of copper, rarefy the gas in the same, and by passing impulses of sufficiently high frequency through a circuit around it, bring the gas inside to a high degree of incandescence; but as to the nature of the forces there would be considerable uncertainty, for it would be doubtful whether with such impulses the copper core would act as a static screen.

Our condensers, which compress, cool, and rarefy air, enabling travellers to obtain water and even ice from the atmosphere, are great aids in desert exploration, removing absolutely the principal distress of the ancient caravan.